The invention refers to a friction lining, in particular for brakes and clutches, as well as to a method of producing the same. The invention further relates to a brake or clutch, as well as to the use of an anode alloy in a friction lining mixture.
Such friction linings are necessary for brakes and clutches, e.g for use in motor vehicles.
Conventional brake lining recipes have the following general structure:
metals (as fibers or powder)
fillers (incl. possible inorganic fibers)
antiseize agents (solid lubricants)
organic components (resins, rubbers, organic fibers, organic fillers).
Depending on the requirements, these four groups of raw materials are present in different proportions.
Important objective of the development of friction material are the optimizing of the friction value in conjunction with the friction partners of the friction lining, the reduction of the wear of the friction partners, and the optimizing of the thermal behavior of the friction partners.
During extended rest periods of vehicles or under rough environmental conditions, e.g. in sea climate, rust often occurs on the iron-containing friction partners, i.e. on the brake disc or the brake drum of a brake system, or on iron-containing clutch elements. For example, rust may cause the friction linings to seize to the frictional counterpart such that the clutch cannot be operated or the brakes cannot be released. Even if the friction linings are not stuck, functional disorders occur. Clutches are not operable as softly as usual. Also the brakes malfunction in operation due to an increased generation of noise during braking. At least as long as the layer of rust has not been removed by abrasion. A further disadvantage is the increased wear of the iron-containing friction partner due to the destruction by rust.
This problem is of particular importance with sea transports of vehicles that are often carried on transport ships for weeks.
From EP-A-0 079 732, it is already known to avoid rust in the steel and cast iron elements of brakes and clutches by coating the friction material with a metal using a flame-spraying method. This metal forms a less noble metal surface than steel and consists of an alloy of either zinc, aluminum or magnesium.
This approach merely prevents rusting during the transport of the vehicles over sea. The thin protective layer will be removed after a few operations of the brakes or the clutch, which is why rust may still occur if the vehicles are further stored in a vehicle yard. The known coating neither solves the problem with cars that often experience extended periods of standstill.
Thus, it is the object of the invention to provide a friction lining having corrosion protection properties.
The invention advantageously provides that the friction material mixture includes an alloy of aluminum and zinc, where the amount of zinc in the alloy is higher than about 1 percent by weight, preferably higher than about 2 percent by weight.
Providing a metal in the friction material mixture that is less noble than iron or steel prevents rusting of the iron or steel friction partner. The aluminum-zinc alloy components in the friction lining form a sacrificial anode so that rusting and, in particular, seizure of the friction partner to the friction lining by rusting can be reliably avoided. As an advantage, the sacrificial anode can always regenerate itself with the wear of the friction lining.
It is another advantage that the friction value can be made uniform by adding the aluminum-zinc alloy.
Corrosion inhibiting particles can become effective only if they are uniformly distributed over the cross section of the friction linings. This is achieved in a particularly favorable manner with an anode alloy of the type AlZn5 which is preferably added as a powder. Here, the term xe2x80x9canode alloyxe2x80x9d refers to a material and an application technique as described in chapter 5.9.2 for the cathodic protection of steel in the Aluminium-Taschenbuch, 15. Ed., page 737.
The amount of zinc in the alloy can be between 2 and 8 percent by weight.
It is particularly preferred to set the amount of zinc in the anode alloy between 4 percent by weight and 6 percent by weight, because the galvanic effect can unfold in an optimum manner for the protection against corrosion.
The electrode potential of an aluminum-zinc alloy drops dramatically with the increase of the amount of zinc and reaches a minimum at an amount of about 5 percent by weight. Therefore, an amount of zinc of about 5 percent by weight is particularly preferred.
Since the friction linings include further components, optimum effects depend on the balancing of the components in the overall mixture. Under this aspect, the present anode alloy has been optimized using the following limits of contents:
zinc 4.5 to 5.5%
tin 0.05 to 0.2%
gallium 0.02 to 0.25%
silicon max. 0.5%
iron max. 0.1%
other additives each max. 0.01% and a total of max. 0.05%, remainder:
aluminum.
The interaction between the optimized anode alloy and the other components maintains the electrochemical effect constant for the entire service life of the friction lining.
The optimized anode alloy comprises an additional amount of tin of 0.05 to 0.2 percent by weight that favorably alters the metal grid of the aluminum matrix, thereby removing passivity problems that otherwise occur in aluminum materials in aqueous electrolytes due to the protective oxide layer that hardly conducts ions and electrons.
The additional amount of 0.02 to 0.25 percent by weight of gallium has a positive, i.e. activating effect on the present anode alloy. The stationary localized corrosion potential of the aluminum anode alloy is thus again decreased so that its passive region is very much restricted and the present anode alloy, while in metal contact with steel, iron, copper, or other aluminum materials, etc., assumes the function of the sacrificial anode, wherein it is used up increasingly.
Another advantage may be achieved with the present anode alloy when it is added to the friction lining as a powder. Adding it as a powder reduces the sticking of iron an steel parts to the friction linings of brakes and clutches that occurs under particular environmental conditions, thereby reducing the so-called xe2x80x9cadhesive corrosionxe2x80x9d.
The weight percentage of the aluminum-zinc alloy components in the friction material mixture may be between 0.5 and 15 percent by weight.
The aluminum-zinc alloy is introduced into the friction material mixture preferably as particles. As a lubricant, tin sulfides in percentage by weight between about 0.5 to 10 percent by weight, preferably about 2 to 8 percent by weight, can be included.
It is provided for manufacturing the friction lining first to liquify the aluminum zinc alloy, preferably present as bars or blocks, and then to atomize it so as to obtain substantially spherical particles. These particles are then mixed with a conventional friction material mixture and pressed to form a friction lining.
It is also possible to form powdery particles directly from the molt of the anode alloy, e.g. by atomizing or by spinning over the rim of a rotating disc.
The following is a detailed description of embodiments of the invention with reference to the accompanying sole drawing.